The field of art of this invention is the one encompassing processes in which hydrogen is produced in elemental form by a chemical reaction between water in any state and a metal or metallic compound. The field is further defined in that the metal or metallic compound which is consumed (i.e. oxidized) in the above reaction is regenerated by reaction with a carbonaceous reducing composition. Thus this field is distinguished from thermochemical processes and from processes in which the metal is regenerated electrolytically. The field is further defined in that the reaction with H.sub.2 O is caused to occur in at least two successive stages, the second stage having a higher equilibrium pH.sub.2 /pH.sub.2 O value than the first.
Typical prior art related to the field of this invention is described in U.S. Pat. Nos. 3,442,620, 3,821,362, and 3,859,373.
Some of the problems existing in the prior art practice of this field of invention are as follows. One prior art process using a successive two-stage reaction with H.sub.2 O has a first stage using the Fe.sub.3 O.sub.4 -FeO equilibrium, and a second using the FeO-Fe equilibrium. All nongaseous species are present in the pure state, i.e. at essentially unit activity, causing the equilibrum pH.sub.2 /pH.sub.2 O of each stage to be uniquely determined by stage temperature. Stage temperature is constrained by other factors: fast enough reaction rate, below agglomeration temperature, process heat balance, etc. Therefore complete equilibrium conversion has not been possible, owing to mass balance effects, and even if equilibrium conversion could be attained, the available values of equilibrium pH.sub.2 /pH.sub.2 O do not necessarily yield the optimum process results achievable in two stages. A third successive stage of oxidation is inherently not possible with pure iron or iron compounds, as the Fe.sub.3 O.sub.4 -Fe.sub.2 O.sub.3 equilibrium has too low of an oxygen affinity. Also all the nongaseous reacting species in the iron process are in solid phase at process conditions, introducing difficult materials handling and transport problems regardless of what type of processing is used: fluidized bed, free fall, fixed bed, etc.